Different phenotypic classes of Sinorhizobium meliloti mutants defective in synthesis of K antigen

For Sinorhizobium meliloti (also known as Rhizobium meliloti) AK631 to establish effective symbiosis with alfalfa, it must be able to synthesize a symbiotically active form of its K antigen, a capsular polysaccharide containing a kdo (3-deoxy-D-manno-octulosonic acid) derivative. Previously isolated mutants defective in the synthesis of K antigen are resistant to bacteriophage phi 16-3. By screening ca. 100,000 TnS-mutagenized R. meliloti bacteria for resistance to bacteriophage phi 16-3, we isolated 119 mutants, 31 of which could not be complemented by genes previously identified as being required for It-antigen synthesis. Of these 31 new mutants, 13 were symbiotically defective and lacked the It antigen. Through genetic and phenotypic analyses, we have grouped these mutants into four distinct classes. Although all of these mutants lack the It antigen, many also have altered Lipopolysaccharides (LPS), suggesting that the biochemical pathways for the synthesis of Ii antigen and LPS have common enzymatic steps. In addition,,ve have found that these and other classes of It-antigen-defective mutants of S. meliloti AK631 exhibit unique patterns of sensitivities to phage strains to which the parental strain was resistant. Our studies have identified new classes of genes required for both the synthesis of K antigen and the symbiotic proficiency of S. meliloti AK631. Some of these classes of genes also play a role in LPS synthesis.